Mobility decisions including cell reselection and handover in Universal Terrestrial Radio Access Networks (UTRAN) and evolved UTRAN (E-UTRAN) rely on downlink measurements performed by the user equipments (UE). Therefore, in order to ensure a good UE mobility performance the UE need to be able to measure and keep track of a certain number of best cells in terms of downlink measured quality.
In UTRAN a UE in active mode is required to measure and to be able to report the downlink quality (i.e. CPICH measurements) of at least 8 cells (one serving and seven neighbour cells), while fulfilling the minimum performance requirements as specified in the document 3GPP TS 25.133, “Requirements for support of radio resource management (FDD)” issued by the 3rd Generation Partnership Project (3GPP). For E-UTRAN, measurement performance requirements are expected to be similar to those used in UTRAN and will most likely be specified in the document 3GPP TS 36.801, “Evolved Universal Terrestrial Radio Access (E-UTRA); Measurement Requirements”.
Basically, two kinds of mobility can be distinguished, whereby both the mobility decisions are mainly based on the same kind of downlink measurements as will be discussed in more details below.                a) Idle mode mobility includes cell reselection, which is mainly a UE-autonomous function without the intervention of its serving cell. However, to some extent the UE behaviour in this mobility scenario could still be controlled by some broadcasted system parameters and performance specification.        b) Connected mode mobility includes handovers, which are fully controlled by the network through explicit UE specific commands and by performance specification. But the handover decisions do heavy rely on the UE measurement reports.        
UTRAN and E-UTRAN are frequency reuse-1 systems meaning that the geographically closest neighbour cells operate on the same carrier frequency. An operator may also deploy multiple frequency layers within the same coverage area. Therefore, idle mode and connected mode mobility in both UTRAN and E-UTRAN can be broadly classified into the following three main categories:                Intra-frequency mobility (idle and connected modes): In intra-frequency mobility a UE moves between the cells belonging to the same carrier frequency. This is the most important mobility scenario since it involves less cost in terms of delay due. In addition an operator would have at least one carrier at its disposal that it would like to be efficiently utilized.        Inter-frequency mobility (idle and connected modes): In inter-frequency mobility the UE moves between cells belonging to different carrier frequencies but of the same access technology. This could be considered as the second most important scenario.        Inter-RAT mobility (idle and connected modes): In inter-RAT mobility the UE moves between cells that belong to different access technologies such as between UTRAN and GSM or vice versa.        
In UTRAN the following three downlink quality measurements are specified, primarily for mobility reasons (cf. 3GPP TS 25.215, “Physical layer measurements (FDD)”): CPICH RSCP, CPICH Ec/No, and UTRA carrier RSSI. The first two of the above measurements are performed by the UE on cell level basis on the common pilot channel (CPICH). The UTRA carrier RSSI is measured over the entire carrier. The above CPICH measurements are the main quantities used for the mobility decisions.
In E-UTRAN the following three downlink quality measurements are specified primarily for mobility reasons (cf. 3GPP TS 36.214 “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer measurements”): Reference symbol received power (RSRP), reference symbol received quality (RSRQ), and E-UTRA carrier RSSI. The first two of the above measurements are performed by the UE on cell level basis on reference symbols. As in case of UTRAN, the E-UTRA carrier RSSI is measured over the entire carrier. The two RS based measurements are indeed also the main quantities, which are likely to be used for the mobility decisions.
In order to guarantee good mobility performance several aspects of measurement performance of the downlink quality measurements have been specified whereby the specification covers the minimum requirements to ensure that a UE meets at least these requirements including                identification delay of unknown cells for the given received level of the corresponding synchronization and CPICH signals. The max delay is up to 800 ms for intra-frequency cells;        minimum number of identified cells (N=8 including one serving and 7 neighbour cells) for which UE is supposed to report the CPICH measurements with the specified measurement absolute and relative accuracies;        the measurement period of 200 ms over which the specified measurement accuracies of at least 8 cells is fulfilled.        
To fulfil the standardized minimum requirements and due to varying radio conditions the UE needs to identify and measure the neighbour cells on regular basis. The scheduling of such measurement process is not standardized but is rather UE implementation specific. To limit hardware costs and to prevent battery exhaustion the UE can typically collect measurement samples for different type of measurements at some periodic intervals.
In idle mode the UE performs measurements mainly at the paging occasions (i.e. at the wake instances at the end of DRX cycle). Therefore, the measurement sampling rate in idle mode is considerably low compared to the connected mode scenario. Thus, measurement performance in idle mode becomes much coarser than in connected mode, but the UE is enabled to save power in idle mode. Furthermore, in idle mode the mobility performance, which is affected by the measurement performance, is required to be less demanding than in connected mode.
In Release 7 of the 3GPP-specifications a new feature called discontinuous reception (DRX) in connected mode (more specifically in CELL_DCH state) has been specified. This feature allows a UE to save its battery while stay connected since it wakes up only at periodic instances according to the DRX cycle. However, the DRX feature also implies that the UE will mainly collect the measurement samples at the wake up instances. Accordingly, the measurement requirements have been relaxed in DRX mode. Therefore, DRX in active mode may have some adverse effect on the mobility performance. In order that the UE can save power, some level of performance degradation is inevitable, but it should be limited to a level which can ensure some minimum quality of service.
The present invention addresses problems in conjunction with existing measurements as described above. Regarding measurements in a non-DRX scenario, the UE performs in the basic intra-frequency mobility scenario in the connected mode without any DRX the downlink measurement on all the desired cells (i.e. 8 cells in UTRAN) with the same intensity. This means the same measurement performance (e.g. measurement delay, cell identification delay, measurement accuracy etc) is achieved for all cells irrespective of their reception quality. This, however, leads to intense processing at the UE whereas in some scenarios it might be sufficient to attain the better performance for a sub-set of the cells. In such cases the processing at the UE could be minimized. Regarding measurements in a DRX scenario, if the UE only measures during the active times the measurement performance of the mobility-related measurements will be worse than in non DRX scenario. The performance degradation depends upon the actual DRX cycle used. For instance, due to the DRX operation the measurement period can be extended resulting in longer measurement reporting delay and, as a consequence, delay handover decisions at the base station, which relies on UE measurement reports for executing handover. Therefore, in active mode the performance degradation of these measurements should be minimized to prevent unnecessary call dropping.
There are several prior-art solutions for minimizing the performance degradation of the measurements. However, the main limitation of these solutions is that they do not consider the relative performance difference between the serving and the target/neighbour cell which is important in mobility performance. The following methods have been proposed:
Reduction in the number of cells: One solution is simply to reduce the number of cells that the UE is supposed to measure in DRX mode, e.g. 4 cells in DRX instead of 8 in non DRX. This would reduce the measurement delay since UE has to sample fewer cells. The main drawback is that UE keeps track of only fewer cells than are needed. The radio conditions could change quickly thereby worse cells could become better and vice versa. For this reason it's advisable that UE measures all the desired number of cells e.g. 8 cells in UTRAN.
Measure on cells with higher SCH and CPICH received levels: The currently agreed solution is that UE in DRX performs measurement on the same number of cells as in case of non DRX provided the minimum received level on synchronization channel (SCH) and common pilot channel (CPICH) on that cell is significantly higher (e.g. 3 dB higher than in case of DRX). Since cells are relatively stronger therefore on the average the measurement delay would be slightly reduced.
Absolute thresholds based measurements: The network provides the UE with absolute thresholds in terms of CPICH Ec/Io or CPICH RSCP levels. As long as the received CPICH Ec/No and/or CPICH RSCP from the serving cell are above these signalled thresholds, the UE is not required to measure other intra-frequency cells. When this condition is no longer met, the UE will start measuring other neighbour cells. A similar mechanism is used in idle mode to save UE battery. One difference between idle and active modes in UTRAN is that in idle mode there is only one cell used by the UE for decoding, whereas in active mode the UE may be receiving channels from multiple cells in soft handover. Thus it is important that the UE keeps track of more than one cell in active mode. Furthermore, in active mode due to the risk of call dropping it is not feasible that the UE performs measurement only on the serving cell until the corresponding measurement quantity falls below the threshold.
Absolute thresholds and measurement activity based measurements: This describes a refinement of scheme described above. The idea is that the network signals CPICH Ec/No and/or CPICH RSCP absolute thresholds and some measurement activity factor. The latter parameter is used to scale the measurement activity (e.g. cell identification time, measurement period) depending on the CPICH Ec/Io and/or CPICH RSCP reception level of the strongest monitored cell in the active set. The UE measures on the target cells with varying activity level that depends on the reception level of the serving cell. One drawback is that the UE will have to measure more often and potentially will have to wake up during the inactive time of the DRX length. This will drain UE battery thereby defying the benefits of DRX operation.
Switch to continuous mode (non DRX) in cell border region: The UTRAN specifications allow network in active mode to promptly direct the UE to switch between DRX and non DRX modes through low level signalling. Thus, one possible solution to speed up the measurement process is to revert to non DRX mode (or very short DRX cycle) when UE enters in cell border region. The network can determine whether UE lies in cell border region or not by comparing the UE reported downlink channel quality (e.g. CPICH level) with a certain threshold. Typically between 25-35% users operate in the cell border region. Thus with approach on the average between 65-75% of the time the UE could barely stay in DRX. Since UE cannot fully utilize DRX, therefore this approach is not desirable from UE battery saving perspective.